Planar cell polarity, ciliogenesis and neural tube defects

Formin Binding Protein 1 (FNBP1) regulates non-canonical Wnt signaling and vertebrate gastrulation (12 words)

The Formin protein Daam1 is required for Wnt-induced cytoskeletal changes during gastrulation, though how it accomplishes this remains unresolved. Here we report the characterization of Formin Binding Protein 1 (FNBP1) as a binding partner of Daam1. The interaction of Daam1 with FNBP1 and its domains required for this interaction were delineated. Immunofluorescence studies showed FNBP1 co-localizes with Daam1, and is an integral component of the actin cytoskeletal complex that is responsive to Wnt stimulation. Specifically, FNBP1 can induce intracellular tubule-like structures and localize to focal adhesions suggesting a role for FNBP1 in cell migration. Functional FNBP1 studies in Xenopus embryos uncover a critical role for FNBP1 in regulating vertebrate gastrulation. Additionally, suboptimal doses of Daam1 and FNBP1 synergize to produce severe gastrulation defects, indicating FNBP1 and Daam1 may function within the same signaling pathway. These results together show FNBP1 is an integral component of Daam1-regulated non-canonical Wnt signaling required for vertebrate gastrulation.

Planar Cell Polarity Signaling: Coordinated Crosstalk for Cell Orientation

The planar cell polarity (PCP) system is essential for positioning cells in 3D networks to establish the proper morphogenesis, structure, and function of organs during embryonic development. The PCP system uses inter- and intracellular feedback interactions between components of the core PCP, characterized by coordinated planar polarization and asymmetric distribution of cell populations inside the cells. PCP signaling connects the anterior-posterior to left-right embryonic plane polarity through the polarization of cilia in the Kupffer's vesicle/node in vertebrates. Experimental investigations on various genetic ablation-based models demonstrated the functions of PCP in planar polarization and associated genetic disorders. This review paper aims to provide a comprehensive overview of PCP signaling history, core components of the PCP signaling pathway, molecular mechanisms underlying PCP signaling, interactions with other signaling pathways, and the role of PCP in organ and embryonic development. Moreover, we will delve into the negative feedback regulation of PCP to maintain polarity, human genetic disorders associated with PCP defects, as well as challenges associated with PCP.

Early life exposure to perfluorooctanesulfonate (PFOS) impacts vital biological processes in Xenopus laevis: Integrated morphometric and transcriptomic analyses

Perfluorooctanesulfonate (PFOS) is a ubiquitous environmental pollutant associated with increasing health concerns and environmental hazards. Toxicological analyses of PFOS exposure are hampered by large interspecies variations and limited studies on the mechanistic details of PFOS-induced toxicity. We investigated the effects of PFOS exposure on Xenopus laevis embryos based on the reported developmental effects in zebrafish. X. laevis was selected to further our understanding of interspecies variation in response to PFOS, and we built upon previous studies by including transcriptomics and an assessment of ciliogenic effects. Midblastula-stage X. laevis embryos were exposed to PFOS using the frog embryo teratogenesis assay Xenopus (FETAX). Results showed teratogenic effects of PFOS in a time- and dose-dependent manner. The morphological abnormalities of skeleton deformities, a small head, and a miscoiled gut were associated with changes in gene expression evidenced by whole-mount in situ hybridization and transcriptomics. The transcriptomic profile of PFOS-exposed embryos indicated the perturbation in the expression of genes associated with cell death, and downregulation in adenosine triphosphate (ATP) biosynthesis. Moreover, we observed the effects of PFOS exposure on cilia development as a reduction in the number of multiciliated cells and changes in the directionality and velocity of the cilia-driven flow. Collectively, these data broaden the molecular understanding of PFOS-induced developmental effects, whereby ciliary dysfunction and disrupted ATP synthesis are implicated as the probable modes of action of embryotoxicity. Furthermore, our findings present a new challenge to understand the links between PFOS-induced developmental toxicity and vital biological processes.

The complex relationship of Wnt-signaling pathways and cilia

Wnt family proteins are secreted glycolipoproteins that signal through multitude of signal transduction pathways. The Wnt-pathways are conserved and critical in all metazoans. They are essential for embryonic development, organogenesis and homeostasis, and associated with many diseases when defective or deregulated. Wnt signaling pathways comprise the canonical Wnt pathway, best known for its stabilization of β-catenin and associated nuclear β-catenin activity in gene regulation, and several non-canonical signaling branches. Wnt-Planar Cell Polarity (PCP) signaling has received the most attention among the non-canonical Wnt pathways. The relationship of cilia to Wnt-signaling is complex. While it was suggested that canonical Wnt signaling requires cilia this notion was always challenged by results suggesting the opposite. Recent developments provide insight and clarification to the relationship of Wnt signaling pathways and cilia. First, it has been now demonstrated that while ciliary proteins, in particular the IFT-A complex, are required for canonical Wnt/β-catenin signaling, the cilium as a structure is not. In contrast, recent work has defined a diverged canonical signaling branch (not affecting β-catenin) to be required for ciliary biogenesis and cilia function. Furthermore, the non-canonical Wnt-PCP pathway does not affect cilia biogenesis per se, but it regulates the position of cilia within cells in many cell types, possibly in all cells where it is active, with cilia being placed near the side of the cell that has the Frizzled-Dishevelled complex. This Wnt/PCP feature is conserved with both centrioles and basal bodies/cilia being positioned accordingly, and it is also used to align mitotic spindles within the Wnt-PCP polarization axis. It also coordinates the alignment of cilia in multiciliated cells. This article addresses these new insights and different links and relationships between cilia and Wnt signaling.

A Van Gogh/Vangl tyrosine phosphorylation switch regulates its interaction with core Planar Cell Polarity factors Prickle and Dishevelled

Epithelial tissues can be polarized along two axes: in addition to apical-basal polarity they are often also polarized within the plane of the epithelium, known as planar cell polarity (PCP). PCP depends upon the conserved Wnt/Frizzled (Fz) signaling factors, including Fz itself and Van Gogh (Vang/Vangl in mammals). Here, taking advantage of the complementary features of Drosophila wing and mouse skin PCP establishment, we dissect how Vang/Vangl phosphorylation on a specific conserved tyrosine residue affects its interaction with two cytoplasmic core PCP factors, Dishevelled (Dsh/Dvl1-3 in mammals) and Prickle (Pk/Pk1-3). We demonstrate that Pk and Dsh/Dvl bind to Vang/Vangl in an overlapping region centered around this tyrosine. Strikingly, Vang/Vangl phosphorylation promotes its binding to Prickle, a key effector of the Vang/Vangl complex, and inhibits its interaction with Dishevelled. Thus phosphorylation of this tyrosine appears to promote the formation of the mature Vang/Vangl-Pk complex during PCP establishment and conversely it inhibits the Vang interaction with the antagonistic effector Dishevelled. Intriguingly, the phosphorylation state of this tyrosine might thus serve as a switch between transient interactions with Dishevelled and stable formation of Vang-Pk complexes during PCP establishment.

Balancing WNT signalling in early forebrain development: The role of LRP4 as a modulator of LRP6 function

The specification of the forebrain relies on the precise regulation of WNT/ß-catenin signalling to support neuronal progenitor cell expansion, patterning, and morphogenesis. Imbalances in WNT signalling activity in the early neuroepithelium lead to congenital disorders, such as neural tube defects (NTDs). LDL receptor-related protein (LRP) family members, including the well-studied receptors LRP5 and LRP6, play critical roles in modulating WNT signalling capacity through tightly regulated interactions with their co-receptor Frizzled, WNT ligands, inhibitors and intracellular WNT pathway components. However, little is known about the function of LRP4 as a potential modulator of WNT signalling in the central nervous system. In this study, we investigated the role of LRP4 in the regulation of WNT signalling during early mouse forebrain development. Our results demonstrate that LRP4 can modulate LRP5- and LRP6-mediated WNT signalling in the developing forebrain prior to the onset of neurogenesis at embryonic stage 9.5 and is therefore essential for accurate neural tube morphogenesis. Specifically, LRP4 functions as a genetic modifier for impaired mitotic activity and forebrain hypoplasia, but not for NTDs in LRP6-deficient mutants. In vivo and in vitro data provide evidence that LRP4 is a key player in fine-tuning WNT signalling capacity and mitotic activity of mouse neuronal progenitors and of human retinal pigment epithelial (hTERT RPE-1) cells. Our data demonstrate the crucial roles of LRP4 and LRP6 in regulating WNT signalling and forebrain development and highlight the need to consider the interaction between different signalling pathways to understand the underlying mechanisms of disease. The findings have significant implications for our mechanistic understanding of how LRPs participate in controlling WNT signalling.

CIC missense variants contribute to susceptibility for spina bifida

Neural tube defects (NTDs) are congenital malformations resulting from abnormal embryonic development of the brain, spine, or spinal column. The genetic etiology of human NTDs remains poorly understood despite intensive investigation. CIC, homolog of the Capicua transcription repressor, has been reported to interact with ataxin-1 (ATXN1) and participate in the pathogenesis of spinocerebellar ataxia type 1. Our previous study demonstrated that CIC loss of function (LoF) variants contributed to the cerebral folate deficiency syndrome by downregulating folate receptor 1 (FOLR1) expression. Given the importance of folate transport in neural tube formation, we hypothesized that CIC variants could contribute to increased risk for NTDs by depressing embryonic folate concentrations. In this study, we examined CIC variants from whole-genome sequencing (WGS) data of 140 isolated spina bifida cases and identified eight missense variants of CIC gene. We tested the pathogenicity of the observed variants through multiple in vitro experiments. We determined that CIC variants decreased the FOLR1 protein level and planar cell polarity (PCP) pathway signaling in a human cell line (HeLa). In a murine cell line (NIH3T3), CIC loss of function variants downregulated PCP signaling. Taken together, this study provides evidence supporting CIC as a risk gene for human NTD.

The role of Lrp6-mediated Wnt/β-catenin signaling in the development and intervention of spinal neural tube defects in mice

Neural tube defects (NTDs) are among the common and severe birth defects with poorly understood etiology. Mutations in the Wnt co-receptor LRP6 are associated with NTDs in humans. Either gain-of-function (GOF) or loss-of-function (LOF) mutations of Lrp6 can cause NTDs in mice. NTDs in Lrp6-GOF mutants may be attributed to altered β-catenin-independent noncanonical Wnt signaling. However, the mechanisms underlying NTDs in Lrp6-LOF mutants and the role of Lrp6-mediated canonical Wnt/β-catenin signaling in neural tube closure remain unresolved. We previously demonstrated that β-catenin signaling is required for posterior neuropore (PNP) closure. In the current study, conditional ablation of Lrp6 in dorsal PNP caused spinal NTDs with diminished activities of Wnt/β-catenin signaling and its downstream target gene Pax3, which is required for PNP closure. β-catenin-GOF rescued NTDs in Lrp6-LOF mutants. Moreover, maternal supplementation of a Wnt/β-catenin signaling agonist reduced the frequency and severity of spinal NTDs in Lrp6-LOF mutants by restoring Pax3 expression. Together, these results demonstrate the essential role of Lrp6-mediated Wnt/β-catenin signaling in PNP closure, which could also provide a therapeutic target for NTD intervention through manipulation of canonical Wnt/β-catenin signaling activities.

Summary: Conditional ablation of Lrp6 in dorsal neural folds causes spinal neural tube defects that can be rescued by genetic activation of β-catenin or maternal supplementation of Wnt signaling agonists.

The regulatory roles of motile cilia in CSF circulation and hydrocephalus

Background

Cerebrospinal fluid (CSF) is an ultra-filtrated colorless brain fluid that circulates within brain spaces like the ventricular cavities, subarachnoid space, and the spine. Its continuous flow serves many primary functions, including nourishment, brain protection, and waste removal.

Main body

The abnormal accumulation of CSF in brain cavities triggers severe hydrocephalus. Accumulating evidence had indicated that synchronized beats of motile cilia (cilia from multiciliated cells or the ependymal lining in brain ventricles) provide forceful pressure to generate and restrain CSF flow and maintain overall CSF circulation within brain spaces. In humans, the disorders caused by defective primary and/or motile cilia are generally referred to as ciliopathies. The key role of CSF circulation in brain development and its functioning has not been fully elucidated.

Conclusions

In this review, we briefly discuss the underlying role of motile cilia in CSF circulation and hydrocephalus. We have reviewed cilia and ciliated cells in the brain and the existing evidence for the regulatory role of functional cilia in CSF circulation in the brain. We further discuss the findings obtained for defective cilia and their potential involvement in hydrocephalus. Furthermore, this review will reinforce the idea of motile cilia as master regulators of CSF movements, brain development, and neuronal diseases.

nNOS regulates ciliated cell polarity, ciliary beat frequency, and directional flow in mouse trachea

Clearance of the airway is dependent on directional mucus flow across the mucociliary epithelium, and deficient flow is implicated in a range of human disorders. Efficient flow relies on proper polarization of the multiciliated cells and sufficient ciliary beat frequency. We show that NO, produced by nNOS in the multiciliated cells of the mouse trachea, controls both the planar polarity and the ciliary beat frequency and is thereby necessary for the generation of the robust flow. The effect of nNOS on the polarity of ciliated cells relies on its interactions with the apical networks of actin and microtubules and involves RhoA activation. The action of nNOS on the beat frequency is mediated by guanylate cyclase; both NO donors and cGMP can augment fluid flow in the trachea and rescue the deficient flow in nNOS mutants. Our results link insufficient availability of NO in ciliated cells to defects in flow and ciliary activity and may thereby explain the low levels of exhaled NO in ciliopathies.

Human myelomeningocele risk and ultra-rare deleterious variants in genes associated with cilium, WNT-signaling, ECM, cytoskeleton and cell migration

Myelomeningocele (MMC) affects one in 1000 newborns annually worldwide and each surviving child faces tremendous lifetime medical and caregiving burdens. Both genetic and environmental factors contribute to disease risk but the mechanism is unclear. This study examined 506 MMC subjects for ultra-rare deleterious variants (URDVs, absent in gnomAD v2.1.1 controls that have Combined Annotation Dependent Depletion score ≥ 20) in candidate genes either known to cause abnormal neural tube closure in animals or previously associated with human MMC in the current study cohort. Approximately 70% of the study subjects carried one to nine URDVs among 302 candidate genes. Half of the study subjects carried heterozygous URDVs in multiple genes involved in the structure and/or function of cilium, cytoskeleton, extracellular matrix, WNT signaling, and/or cell migration. Another 20% of the study subjects carried heterozygous URDVs in candidate genes associated with gene transcription regulation, folate metabolism, or glucose metabolism. Presence of URDVs in the candidate genes involving these biological function groups may elevate the risk of developing myelomeningocele in the study cohort.

A null allele of Dnaaf2 displays embryonic lethality and mimics human ciliary dyskinesia

The dynein axonemal assembly factor (Dnaaf) protein family is involved in preassembly and stability of dynein arms before they are transported into the cilia. In humans, mutations in DNAAF genes lead to several diseases related to cilia defects such as primary ciliary dyskinesia (PCD; OMIM: 612518). Patients with PCD experience malfunctions in cilia motility, which can result in inflammation and infection of the respiratory tract among other defects. Previous studies have identified that a mutation in DNAAF2 results in PCD and that 40% of these patients also experience laterality defects. In an outbred genetic background, Dnaaf2 homozygotes die after birth and have left/right defects among other phenotypes. Here we characterize a novel null allele of Dnaaf2 obtained from the International Mouse Phenotyping Consortium. Our data indicate that on a defined C57bl/6NJ genetic background, homozygous Dnaaf2 mouse embryos fail to progress beyond organogenesis stages with many abnormalities including left-right patterning defects. These findings support studies indicating that hypomorphic mutations of human DNAAF2 can result in ciliary dyskinesia and identify Dnaaf2 as an essential component of cilia function in vivo.

Ciliary and cytoskeletal functions of an ancient monooxygenase essential for bioactive amidated peptide synthesis

Many secreted peptides used for cell-cell communication require conversion of a C-terminal glycine to an amide for bioactivity. This reaction is catalyzed only by the integral membrane protein peptidylglycine α-amidating monooxygenase (PAM). PAM has been highly conserved and is found throughout the metazoa; PAM-like sequences are also present in choanoflagellates, filastereans, unicellular and colonial chlorophyte green algae, dinoflagellates and haptophytes. Recent studies have revealed that in addition to playing a key role in peptidergic signaling, PAM also regulates ciliogenesis in vertebrates, planaria and chlorophyte algae, and is required for the stability of actin-based microvilli. Here we briefly introduce the basic principles involved in ciliogenesis, the sequential reactions catalyzed by PAM and the trafficking of PAM through the secretory and endocytic pathways. We then discuss the multi-faceted roles this enzyme plays in the formation and maintenance of cytoskeleton-based cellular protrusions and propose models for how PAM protein and amidating activity might contribute to ciliogenesis. Finally, we consider why some ciliated organisms lack PAM, and discuss the potential ramifications of ciliary localized PAM for the endocrine features commonly observed in patients with ciliopathies.

A deficiency in SUMOylation activity disrupts multiple pathways leading to neural tube and heart defects in Xenopus embryos

Background

Adenovirus protein, Gam1, triggers the proteolytic destruction of the E1 SUMO-activating enzyme. Microinjection of an empirically determined amount of Gam1 mRNA into one-cell Xenopus embryos can reduce SUMOylation activity to undetectable, but nonlethal, levels, enabling an examination of the role of this post-translational modification during early vertebrate development.

Results

We find that SUMOylation-deficient embryos consistently exhibit defects in neural tube and heart development. We have measured differences in gene expression between control and embryos injected with Gam1 mRNA at three developmental stages: early gastrula (immediately following the initiation of zygotic transcription), late gastrula (completion of the formation of the three primary germ layers), and early neurula (appearance of the neural plate). Although changes in gene expression are widespread and can be linked to many biological processes, three pathways, non-canonical Wnt/PCP, snail/twist, and Ets-1, are especially sensitive to the loss of SUMOylation activity and can largely account for the predominant phenotypes of Gam1 embryos. SUMOylation appears to generate different pools of a given transcription factor having different specificities with this post-translational modification involved in the regulation of more complex, as opposed to housekeeping, processes.

Conclusions

We have identified changes in gene expression that underlie the neural tube and heart phenotypes resulting from depressed SUMOylation activity. Notably, these developmental defects correspond to the two most frequently occurring congenital birth defects in humans, strongly suggesting that perturbation of SUMOylation, either globally or of a specific protein, may frequently be the origin of these pathologies.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5773-3) contains supplementary material, which is available to authorized users.

Neural control of body-plan axis in regenerating planaria

Control of axial polarity during regeneration is a crucial open question. We developed a quantitative model of regenerating planaria, which elucidates self-assembly mechanisms of morphogen gradients required for robust body-plan control. The computational model has been developed to predict the fraction of heteromorphoses expected in a population of regenerating planaria fragments subjected to different treatments, and for fragments originating from different regions along the anterior-posterior and medio-lateral axis. This allows for a direct comparison between computational and experimental regeneration outcomes. Vector transport of morphogens was identified as a fundamental requirement to account for virtually scale-free self-assembly of the morphogen gradients observed in planarian homeostasis and regeneration. The model correctly describes altered body-plans following many known experimental manipulations, and accurately predicts outcomes of novel cutting scenarios, which we tested. We show that the vector transport field coincides with the alignment of nerve axons distributed throughout the planarian tissue, and demonstrate that the head-tail axis is controlled by the net polarity of neurons in a regenerating fragment. This model provides a comprehensive framework for mechanistically understanding fundamental aspects of body-plan regulation, and sheds new light on the role of the nervous system in directing growth and form.

G-protein-coupled receptor signaling and neural tube closure defects

Disruption of the normal mechanisms that mediate neural tube closure can result in neural tube defects (NTDs) with devastating consequences in affected patients. With the advent of next-generation sequencing, we are increasingly detecting mutations in multiple genes in NTD cases. However, our ability to determine which of these genes contribute to the malformation is limited by our understanding of the pathways controlling neural tube closure. G-protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors in humans and have been historically favored as drug targets. Recent studies implicate several GPCRs and downstream signaling pathways in neural tube development and closure. In this review, we will discuss our current understanding of GPCR signaling pathways in pathogenesis of NTDs. Notable examples include the orphan primary cilia-localized GPCR, Gpr161 that regulates the basal suppression machinery of sonic hedgehog pathway by means of activation of cAMP-protein kinase A signaling in the neural tube, and protease-activated receptors that are activated by a local network of membrane-tethered proteases during neural tube closure involving the surface ectoderm. Understanding the role of these GPCR-regulated pathways in neural tube development and closure is essential toward identification of underlying genetic causes to prevent NTDs. Birth Defects Research 109:129-139, 2017. © 2016 Wiley Periodicals, Inc.

La-related protein 6 controls ciliated cell differentiation

Background

La-related protein 6 (LARP6) is an evolutionally conserved RNA-binding protein. Vertebrate LARP6 binds the 5′ stem-loop found in mRNAs encoding type I collagen to regulate their translation, but other target mRNAs and additional functions for LARP6 are unknown. The aim of this study was to elucidate an additional function of LARP6 and to evaluate the importance of its function during development.

Methods

To uncover the role of LARP6 in development, we utilized Morpholino Oligos to deplete LARP6 protein in Xenopus embryos. Then, embryonic phenotypes and ciliary structures of LAPR6 morphants were examined. To identify the molecular mechanism underlying ciliogenesis regulated by LARP6, we tested the expression level of cilia-related genes, which play important roles in ciliogenesis, by RT-PCR or whole mount in situ hybridization (WISH).

Results

We knocked down LARP6 in Xenopus embryos and found neural tube closure defects. LARP6 mutant, which compromises the collagen synthesis, could rescue these defects. Neural tube closure defects are coincident with lack of cilia, antenna-like cellular organelles with motility- or sensory-related functions, in the neural tube. The absence of cilia at the epidermis was also observed in LARP6 morphants, and this defect was due to the absence of basal bodies which are formed from centrioles and required for ciliary assembly. In the process of multi-ciliated cell (MCC) differentiation, mcidas, which activates the transcription of genes required for centriole formation during ciliogenesis, could partially restore MCCs in LARP6 morphants. In addition, LARP6 likely controls the expression of mcidas in a Notch-independent manner.

Conclusions

La-related protein 6 is involved in ciliated cell differentiation during development by controlling the expression of cilia-related genes including mcidas. This LARP6 function involves a mechanism that is distinct from its established role in binding to collagen mRNAs and regulating their translation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13630-017-0047-7) contains supplementary material, which is available to authorized users.

Fetal DNA hypermethylation in tight junction pathway is associated with neural tube defects: A genome-wide DNA methylation analysis

Neural tube defects (NTDs) are a spectrum of severe congenital malformations of fusion failure of the neural tube during early embryogenesis. Evidence on aberrant DNA methylation in NTD development remains scarce, especially when exposure to environmental pollutant is taken into consideration. DNA methylation profiling was quantified using the Infinium HumanMethylation450 array in neural tissues from 10 NTD cases and 8 non-malformed controls (stage 1). Subsequent validation was performed using a Sequenom MassARRAY system in neural tissues from 20 NTD cases and 20 non-malformed controls (stage 2). Correlation analysis of differentially methylated CpG sites in fetal neural tissues and polycyclic aromatic hydrocarbons concentrations in fetal neural tissues and maternal serum was conducted. Differentially methylated CpG sites of neural tissues were further validated in fetal mice with NTDs induced by benzo(a)pyrene given to pregnant mice. Differentially hypermethylated CpG sites in neural tissues from 17 genes and 6 pathways were identified in stage 1. Subsequently, differentially hypermethylated CpG sites in neural tissues from 6 genes (BDKRB2, CTNNA1, CYFIP2, MMP7, MYH2, and TIAM2) were confirmed in stage 2. Correlation analysis showed that methylated CpG sites in CTNNA1 and MYH2 from NTD cases were positively correlated to polycyclic aromatic hydrocarbon level in fetal neural tissues and maternal serum. The correlation was confirmed in NTD-affected fetal mice that were exposed to benzo(a)pyrene in utero. In conclusion, hypermethylation of the CTNNA1 and MYH2 genes in tight junction pathway is associated with the risk for NTDs, and the DNA methylation aberration may be caused by exposure to benzo(a)pyrene.

Digenic mutations of human OCRL paralogs in Dent's disease type 2 associated with Chiari I malformation

OCRL1 and its paralog INPP5B encode phosphatidylinositol 5-phosphatases that localize to the primary cilium and have roles in ciliogenesis. Mutations in OCRL1 cause the X-linked Dent disease type 2 (DD2; OMIM# 300555), characterized by low-molecular weight proteinuria, hypercalciuria, and the variable presence of cataracts, glaucoma and intellectual disability without structural brain anomalies. Disease-causing mutations in INPP5B have not been described in humans. Here, we report the case of an 11-year-old boy with short stature and an above-average IQ; severe proteinuria, hypercalciuria and osteopenia resulting in a vertebral compression fracture; and Chiari I malformation with cervico-thoracic syringohydromyelia requiring suboccipital decompression. Sequencing revealed a novel, de novo DD2-causing 462 bp deletion disrupting exon 3 of OCRL1 and a maternally inherited, extremely rare (ExAC allele frequency 8.4×10⁻⁶) damaging missense mutation in INPP5B (p.A51V). This mutation substitutes an evolutionarily conserved amino acid in the protein’s critical PH domain. In silico analyses of mutation impact predicted by SIFT, PolyPhen2, MetaSVM and CADD algorithms were all highly deleterious. Together, our findings report a novel association of DD2 with Chiari I malformation and syringohydromyelia, and document the effects of digenic mutation of human OCRL paralogs. These findings lend genetic support to the hypothesis that impaired ciliogenesis may contribute to the development of Chiari I malformation, and implicates OCRL-dependent PIP₃ metabolism in this mechanism.

Boundary propagation of planar cell polarity is robust against cell packing pattern

Planar cell polarity is an important property of epithelial tissue. The boundary propagation model was proposed as the mechanism of PCP induction, while it has been doubted whether it can induce PCP on wide tissue. Using simulation, a set of proteins can be shown to induce PCP, but it does not explain why and how the set can induce PCP. In this study, we made theoretical model and simulation model to explore when and how the boundary propagation induce PCP. We incorporated multipolar cell in our model. Intracellular interactions have been thought to amplify polarity of a cell, but we propose instead that they are to keep a cell-cell interface polarized, and bipolarity of cell is obtained as a result. We show that the boundary propagation can propagate polarity as long as average size of local cell group is constant and levels of PCP proteins are balanced in every cell. Therefore, this model provide an explanation for PCP induction on a tissue with multiple cell types.

Adverse effect of valproic acid on an in vitro gastrulation model entails activation of retinoic acid signaling

Valproic acid (VPA), an antiepileptic drug, is a teratogen that causes neural tube and axial skeletal defects, although the mechanisms are not fully understood. We previously established a gastrulation model using mouse P19C5 stem cell embryoid bodies (EBs), which exhibits axial patterning and elongation morphogenesis in vitro. Here, we investigated the effects of VPA on the EB axial morphogenesis to gain insights into its teratogenic mechanisms. Axial elongation and patterning of EBs were inhibited by VPA at therapeutic concentrations. VPA elevated expression levels of various developmental regulators, including Cdx1 and Hoxa1, known transcriptional targets of retinoic acid (RA) signaling. Co-treatment of EBs with VPA and BMS493, an RA receptor antagonist, partially rescued axial elongation as well as gene expression profiles. These results suggest that VPA requires active RA signaling to interfere with EB morphogenesis.

The E3 ubiquitin ligase Hace1 is required for early embryonic development in Xenopus laevis

BACKGROUND

HECT domain and ankyrin repeat containing E3 ubiquitin protein ligase 1 (HACE1) regulates a wide variety of cellular processes. It has been shown that one of the targets of HACE1 is the GTP-bound form of the small GTPase Rac1. However, the role of HACE1 in early development remains unknown.

RESULTS

In situ hybridization revealed that Xenopus laevis hace1 is specifically expressed in the ectoderm at the blastula and gastrula stages and in the epidermis, branchial arch, kidney, and central nervous system at the tailbud stage. Knockdown of hace1 in Xenopus laevis embryos via antisense morpholino oligonucleotides led to defects in body axis elongation, pigment formation, and eye formation at the tadpole stage. Experiments with Keller sandwich explants showed that hace1 knockdown inhibited convergent extension, a morphogenetic movement known to be crucial for body axis elongation. In addition, time lapse imaging of whole embryos during the neurula stage indicated that hace1 knockdown also delayed neural tube closure. The defects caused by hace1 knockdown were partly rescued by knockdown of rac1. Moreover, embryos expressing a constitutively active form of Rac1 displayed phenotypes similar to those of embryos with hace1 knocked down.

CONCLUSIONS

Our results suggest that Xenopus laevis hace1 plays an important role in early embryonic development, possibly via regulation of Rac1 activity.

Frizzled-Induced Van Gogh Phosphorylation by CK1ε Promotes Asymmetric Localization of Core PCP Factors in Drosophila

Epithelial tissues are polarized along two axes. In addition to apical-basal polarity, they are often polarized within the plane of the epithelium, so-called Planar Cell Polarity (PCP). PCP depends upon Wnt/Frizzled (Fz) signaling factors, including Fz itself and Van Gogh (Vang/Vangl). We sought to understand how Vang interaction with other core PCP factors affects Vang function. We find that Fz induces Vang phosphorylation in a cell-autonomous manner. Vang phosphorylation occurs on conserved N-terminal serine/threonine residues, is mediated by CK1ε/Dco, and is critical for polarized membrane localization of Vang and other PCP proteins. This regulatory mechanism does not require Fz signaling through Dishevelled and thus represents a cell-autonomous upstream interaction between Fz and Vang. Furthermore, this signaling event appears to be related to Wnt5a-mediated Vangl2 phosphorylation during mouse limb patterning and may thus be a general mechanism underlying Wnt-regulated PCP establishment.

MEKK4 Signaling Regulates Sensory Cell Development and Function in the Mouse Inner Ear

Mechanosensory hair cells (HCs) residing in the inner ear are critical for hearing and balance. Precise coordination of proliferation, sensory specification, and differentiation during development is essential to ensure the correct patterning of HCs in the cochlear and vestibular epithelium. Recent studies have revealed that FGF20 signaling is vital for proper HC differentiation. However, the mechanisms by which FGF20 signaling promotes HC differentiation remain unknown. Here, we show that mitogen-activated protein 3 kinase 4 (MEKK4) expression is highly regulated during inner ear development and is critical to normal cytoarchitecture and function. Mice homozygous for a kinase-inactive MEKK4 mutation exhibit significant hearing loss. Lack of MEKK4 activity in vivo also leads to a significant reduction in the number of cochlear and vestibular HCs, suggesting that MEKK4 activity is essential for overall development of HCs within the inner ear. Furthermore, we show that loss of FGF20 signaling in vivo inhibits MEKK4 activity, whereas gain of Fgf20 function stimulates MEKK4 expression, suggesting that Fgf20 modulates MEKK4 activity to regulate cellular differentiation. Finally, we demonstrate, for the first time, that MEKK4 acts as a critical node to integrate FGF20-FGFR1 signaling responses to specifically influence HC development and that FGFR1 signaling through activation of MEKK4 is necessary for outer hair cell differentiation. Collectively, this study provides compelling evidence of an essential role for MEKK4 in inner ear morphogenesis and identifies the requirement of MEKK4 expression in regulating the specific response of FGFR1 during HC development and FGF20/FGFR1 signaling activated MEKK4 for normal sensory cell differentiation.

SIGNIFICANCE STATEMENT

Sensory hair cells (HCs) are the mechanoreceptors within the inner ear responsible for our sense of hearing. HCs are formed before birth, and mammals lack the ability to restore the sensory deficits associated with their loss. In this study, we show, for the first time, that MEKK4 signaling is essential for the development of normal cytoarchitecture and hearing function as MEKK4 signaling-deficient mice exhibit a significant reduction of HCs and a hearing loss. We also identify MEKK4 as a critical hub kinase for FGF20-FGFR1 signaling to induce HC differentiation in the mammalian cochlea. These results reveal a new paradigm in the regulation of HC differentiation and provide significant new insights into the mechanism of Fgf signaling governing HC formation.

The Dishevelled Protein Family: Still Rather a Mystery After Over 20 Years of Molecular Studies

Dishevelled (Dsh) is a key component of Wnt-signaling pathways and possibly also has other functional requirements. Dsh appears to be a key factor to interpret Wnt signals coming via the Wnt-receptor family, the Frizzled proteins, from the plasma membrane and route them into the correct intracellular pathways. However, how Dsh is regulated to relay signal flow to specific and distinct cellular responses upon interaction with the same Wnt-receptor family remains very poorly understood.

Ror2 receptor mediates Wnt11 ligand signaling and affects convergence and extension movements in zebrafish

The receptor-tyrosine kinase Ror2 acts as an alternative receptor or co-receptor for Wnt5a and mediates Wnt5a-induced convergent extension movements during embryogenesis in mice and Xenopus as well as the polarity and migration of several cell types during development. However, little is known about whether Ror2 function is conserved in other vertebrates or is involved in other non-canonical Wnt ligands in vivo. In this study we demonstrated that overexpression of dominant-negative ror2 (ror2-TM) mRNA in zebrafish embryos resulted in convergence and extension defects and incompletely separated eyes, which is consistent with observations from slb/wnt11 mutants or wnt11 knockdown morphants. Moreover, the co-injection of ror2-TM mRNA and a wnt11 morpholino or the coexpression of ror2 and wnt11 in zebrafish embryos synergetically induced more severe convergence and extension defects. Transplantation studies further demonstrated that the Ror2 receptor responded to the Wnt11 ligand and regulated cell migration and cell morphology during gastrulation. DnRor2 inhibited the action of Wnt11, which was revealed by a decreased percentage of Wnt11-induced convergence and extension defects. Ror2 physically interacts with Wnt11. Theintracellular Tyr-647andSer-863 sites ofRor2are essential for mediating the action of Wnt11. Dishevelled and RhoA act downstream of Wnt11-Ror2 to regulate convergence and extension movements. Overall, our data suggest an important role of Ror2 in mediating Wnt11 signaling and in regulating convergence and extension movements in zebrafish.

Cell polarity and neurogenesis in embryonic stem cell-derived neural rosettes

Embryonic stem cells (ESCs) undergoing neural differentiation form radial arrays of neural stem cells, termed neural rosettes. These structures manifest many of the properties associated with embryonic and adult neurogenesis, including cell polarization, interkinetic nuclear migration (INM), and a gradient of neuronal differentiation. We now identify novel rosette structural features that serve to localize key regulators of neurogenesis. Cells within neural rosettes have specialized basal as well as apical surfaces, based on localization of the extracellular matrix receptor β1 integrin. Apical processes of cells in mature rosettes terminate at the lumen, where adherens junctions are apparent. Primary cilia are randomly distributed in immature rosettes and tightly associated with the neural stem cell's apical domain as rosettes mature. Components of two signaling pathways known to regulate neurogenesis in vivo and in rosettes, Hedgehog and Notch, are apically localized, with the Hedgehog effector Smoothened (Smo) associated with primary cilia and the Notch pathway γ-secretase subunit Presenilin 2 associated with the adherens junction. Increased neuron production upon treatment with the Notch inhibitor DAPT suggests a major role for Notch signaling in maintaining the neural stem cell state, as previously described. A less robust outcome was observed with manipulation of Hedgehog levels, though consistent with a role in neural stem cell survival or proliferation. Inhibition of both pathways resulted in an additive effect. These data support a model by which cells extending a process to the rosette lumen maintain neural stem cell identity whereas release from this association, either through asymmetric cell division or apical abscission, promotes neuronal differentiation.

Function and regulation of primary cilia and intraflagellar transport proteins in the skeleton

Primary cilia are microtubule-based organelles that project from the cell surface to enable transduction of various developmental signaling pathways. The process of intraflagellar transport (IFT) is crucial for the building and maintenance of primary cilia. Ciliary dysfunction has been found in a range of disorders called ciliopathies, some of which display severe skeletal dysplasias. In recent years, interest has grown in uncovering the function of primary cilia/IFT proteins in bone development, mechanotransduction, and cellular regulation. We summarize recent advances in understanding the function of cilia and IFT proteins in the regulation of cell differentiation in osteoblasts, osteocytes, chondrocytes, and mesenchymal stem cells (MSCs). We also discuss the mechanosensory function of cilia and IFT proteins in bone cells, cilia orientation, and other functions of cilia in chondrocytes.

Mechanisms of planar cell polarity establishment in Drosophila

Correct patterning and polarization of epithelial and mesenchymal cells are essential for morphogenesis and function of all organs and organisms. Epithelial cells are generally polarized in two axes: (a) the ubiquitous apical-basal axis and (b) polarity within the plane of the epithelium. The latter is generally referred to as planar cell polarity (PCP) and also is found in several contexts of mesenchymal cell patterning. In Drosophila, all adult structures display PCP features, and two conserved molecular systems (the Fat [Ft]/Dachsous [Ds] system and the Frizzled [Fz]/PCP pathway) that regulate this process have been identified. Although significant progress has been made in dissecting aspects of PCP signaling within cells, much remains to be discovered about the mechanisms of long-range and local PCP cell-cell interactions. Here, we discuss the current models based on Drosophila studies and incorporate recent insights into this long-standing cell and developmental biology problem.

Null and hypomorph Prickle1 alleles in mice phenocopy human Robinow syndrome and disrupt signaling downstream of Wnt5a

Planar cell polarity (PCP) signaling plays a critical role in tissue morphogenesis. In mammals, disruption of three of the six "core PCP" components results in polarity-dependent defects with rotated cochlear hair cell stereocilia and open neural tube. We recently demonstrated a role of Prickle1, a core PCP molecule in Drosophila, in mammalian neuronal development. To examine Prickle1 function along a broader developmental window, we generated three mutant alleles in mice. We show that the complete loss of Prickle1 leads to systemic tissue outgrowth defects, aberrant cell organization and disruption of polarity machinery. Curiously, Prickle1 mutants recapitulate the characteristic features of human Robinow syndrome and phenocopy mouse mutants with Wnt5a or Ror2 gene defects, prompting us to explore an association of Prickle1 with the Wnt pathway. We show that Prickle1 is a proteasomal target of Wnt5a signaling and that Dvl2, a target of Wnt5a signaling, is misregulated in Prickle1 mutants. Our studies implicate Prickle1 as a key component of the Wnt-signaling pathway and suggest that Prickle1 mediates some of the WNT5A-associated genetic defects in Robinow syndrome.

Blastopathies and microcephaly in a Chornobyl impacted region of Ukraine

This population-based descriptive epidemiology study demonstrates that rates of conjoined twins, teratomas, neural tube defects, microcephaly, and microphthalmia in the Rivne province of Ukraine are among the highest in Europe. The province is 200 km distant from the Chornobyl site and its northern half, a region known as Polissia, is significantly polluted by ionizing radiation. The rates of neural tube defects, microcephaly and microphthalmia in Polissia are statistically significantly higher than in the rest of the province. A survey of at-birth head size showed that values were statistically smaller in males and females born in one Polissia county than among neonates born in the capital city. These observations provide clues for confirmatory and cause-effect prospective investigations. The strength of this study stems from a reliance on international standards prevalent in Europe and a decade-long population-based surveillance of congenital malformations in two distinct large populations. The limitations of this study, as those of other descriptive epidemiology investigations, is that identified cause-effect associations require further assessment by specific prospective investigations designed to address specific teratogenic factors.

Proteomic profiling of cardiac tissue by isolation of nuclei tagged in specific cell types (INTACT)

The proper dissection of the molecular mechanisms governing the specification and differentiation of specific cell types requires isolation of pure cell populations from heterogeneous tissues and whole organisms. Here, we describe a method for purification of nuclei from defined cell or tissue types in vertebrate embryos using INTACT (isolation of nuclei tagged in specific cell types). This method, previously developed in plants, flies and worms, utilizes in vivo tagging of the nuclear envelope with biotin and the subsequent affinity purification of the labeled nuclei. In this study we successfully purified nuclei of cardiac and skeletal muscle from Xenopus using this strategy. We went on to demonstrate the utility of this approach by coupling the INTACT approach with liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomic methodologies to profile proteins expressed in the nuclei of developing hearts. From these studies we have identified the Xenopus orthologs of 12 human proteins encoded by genes, which when mutated in human lead to congenital heart disease. Thus, by combining these technologies we are able to identify tissue-specific proteins that are expressed and required for normal vertebrate organ development.

Structure and function of the ependymal barrier and diseases associated with ependyma disruption

The neuroepithelium is a germinal epithelium containing progenitor cells that produce almost all of the central nervous system cells, including the ependyma. The neuroepithelium and ependyma constitute barriers containing polarized cells covering the embryonic or mature brain ventricles, respectively; therefore, they separate the cerebrospinal fluid that fills cavities from the developing or mature brain parenchyma. As barriers, the neuroepithelium and ependyma play key roles in the central nervous system development processes and physiology. These roles depend on mechanisms related to cell polarity, sensory primary cilia, motile cilia, tight junctions, adherens junctions and gap junctions, machinery for endocytosis and molecule secretion, and water channels. Here, the role of both barriers related to the development of diseases, such as neural tube defects, ciliary dyskinesia, and hydrocephalus, is reviewed.

Phospho-NHE3 forms membrane patches and interacts with beta-actin to sense and maintain constant direction during cell migration

The Na(+)/H(+) exchanger NHE3 colocalizes with beta-actin at the leading edge of directionally migrating cells. Using human osteosarcoma cells (SaOS-2), rat osteoblasts (calvaria), and human embryonic kidney (HEK) cells, we identified a novel role for NHE3 via beta-actin in anode and cathode directed motility, during electrotaxis. NHE3 knockdown by RNAi revealed that NHE3 expression is required to achieve constant directionality and polarity in migrating cells. Phosphorylated NHE3 (pNHE3) and beta-actin complex formation was impaired by the NHE3 inhibitor S3226 (IC50 0.02µM). Fluorescence cross-correlation spectroscopy (FCCS) revealed that the molecular interactions between NHE3 and beta-actin in membrane protrusions increased 1.7-fold in the presence of a directional cue and decreased 3.3-fold in the presence of cytochalasin D. Data from flow cytometric analysis showed that membrane potential of cells (Vmem) decreases in directionally migrating, NHE3-deficient osteoblasts and osteosarcoma cells whereas only Vmem of wild type osteoblasts is affected during directional migration. These findings suggest that pNHE3 has a mechanical function via beta-actin that is dependent on its physiological activity and Vmem. Furthermore, phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels increase while PIP2 remains stable when cells have persistent directionality. Both PI3 kinase (PI3K) and Akt expression levels change proportionally to NHE3 levels. Interestingly, however, the content of pNHE3 level does not change when PI3K/Akt is inhibited. Therefore, we conclude that NHE3 can act as a direction sensor for cells and that NHE3 phosphorylation in persistent directional cell migration does not involve PI3K/Akt during electrotaxis.

Regulation of PCP by the Fat signaling pathway

Planar cell polarity (PCP) in epithelia, orthogonal to the apical-basal axis, is essential for numerous developmental events and physiological functions. Drosophila model systems have been at the forefront of studies revealing insights into mechanisms regulating PCP and have revealed distinct signaling modules. One of these, involving the atypical cadherins Fat and Dachsous and the ectokinase Four-jointed, appears to link the direction of cell polarization to the tissue axes. We discuss models for the function of this signaling module as well as several unanswered questions that may guide future investigations.

Genetic association analyses of nitric oxide synthase genes and neural tube defects vary by phenotype

Neural tube defects (NTDs) are caused by improper neural tube closure during the early stages of embryonic development. NTDs are hypothesized to have a complex genetic origin and numerous candidate genes have been proposed. The nitric oxide synthase 3 (NOS3) G594T polymorphism has been implicated in risk for spina bifida, and interactions between that single nucleotide polymorphism (SNP) and the methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism have also been observed. To evaluate other genetic variation in the NO pathway in the development of NTDs, we examined all three NOS genes: NOS1, NOS2, and NOS3. Using 3109 Caucasian samples in 745 families, we evaluated association in the overall dataset and within specific phenotypic subsets. Haplotype tagging SNPs in the NOS genes were tested for genetic association with NTD subtypes, both for main effects as well as for the presence of interactions with the MTHFR C677T polymorphism. Nominal main effect associations were found with all subtypes, across all three NOS genes, and interactions were observed between SNPs in all three NOS genes and MTHFR C677T. Unlike the previous report, the most significant associations in our dataset were with cranial subtypes and the AG genotype of rs4795067 in NOS2 (p = 0.0014) and the interaction between the rs9658490 G allele in NOS1 and MTHFR 677TT genotype (p = 0.0014). Our data extend the previous findings by implicating a role for all three NOS genes, independently and through interactions with MTHFR, in risk not only for spina bifida, but all NTD subtypes.

β-catenin regulates Pax3 and Cdx2 for caudal neural tube closure and elongation

Non-canonical Wnt/planar cell polarity (PCP) signaling plays a primary role in the convergent extension that drives neural tube closure and body axis elongation. PCP signaling gene mutations cause severe neural tube defects (NTDs). However, the role of canonical Wnt/β-catenin signaling in neural tube closure and NTDs remains poorly understood. This study shows that conditional gene targeting of β-catenin in the dorsal neural folds of mouse embryos represses the expression of the homeobox-containing genes Pax3 and Cdx2 at the dorsal posterior neuropore (PNP), and subsequently diminishes the expression of the Wnt/β-catenin signaling target genes T, Tbx6 and Fgf8 at the tail bud, leading to spina bifida aperta, caudal axis bending and tail truncation. We demonstrate that Pax3 and Cdx2 are novel downstream targets of Wnt/β-catenin signaling. Transgenic activation of Pax3 cDNA can rescue the closure defect in the β-catenin mutants, suggesting that Pax3 is a key downstream effector of β-catenin signaling in the PNP closure process. Cdx2 is known to be crucial in posterior axis elongation and in neural tube closure. We found that Cdx2 expression is also repressed in the dorsal PNPs of Pax3-null embryos. However, the ectopically activated Pax3 in the β-catenin mutants cannot restore Cdx2 mRNA in the dorsal PNP, suggesting that the presence of both β-catenin and Pax3 is required for regional Cdx2 expression. Thus, β-catenin signaling is required for caudal neural tube closure and elongation, acting through the transcriptional regulation of key target genes in the PNP.

Calpain2 protease: A new member of the Wnt/Ca(2+) pathway modulating convergent extension movements in Xenopus

Calpains are a family of calcium-dependent intracellular cysteine proteases that regulate several physiological processes by limited cleavage of different substrates. The role of Calpain2 in embryogenesis is not clear with conflicting evidence from a number of mouse knockouts. Here we report the temporal and spatial expression of Calpain2 in Xenopus laevis embryos and address its role in Xenopus development. We show that Calpain2 is expressed maternally with elevated expression in neural tissues and that Calpain2 activity is spatially and temporally regulated. Using a Calpain inhibitor, a dominant negative and a morpholino oligonoucleotide we demonstrate that impaired Calpain2 activity results in defective convergent extension both in mesodermal and neural tissues. Specifically, Calpain2 downregulation results in loss of tissue polarity and blockage of mediolateral intercalation in Keller explants without affecting adherens junction turnover. We further show that Calpain2 is activated in response to Wnt5a and that the inhibitory effect of Wnt5a expression on animal cap elongation can be rescued by blocking Calpain2 function. This suggests that Calpain2 activity needs to be tightly regulated during convergent extension. Finally we show that expression of Xdd1 blocks the membrane translocation of Calpain2 suggesting that Calpain2 activation is downstream of Dishevelled. Overall our data show that Calpain2 activation through the Wnt/Ca(2+) pathway and Dishevelled can modulate convergent extension movements.

Novel mechanisms that pattern and shape the midbrain-hindbrain boundary

The midbrain-hindbrain boundary (MHB) is a highly conserved vertebrate signalling centre, acting to pattern and establish neural identities within the brain. While the core signalling pathways regulating MHB formation have been well defined, novel genetic and mechanistic processes that interact with these core components are being uncovered, helping to further elucidate the complicated networks governing MHB specification, patterning and shaping. Although formation of the MHB organiser is traditionally thought of as comprising three stages, namely positioning, induction and maintenance, we propose that a fourth stage, morphogenesis, should be considered as an additional stage in MHB formation. This review will examine evidence for novel factors regulating the first three stages of MHB development and will explore the evidence for regulation of MHB morphogenesis by non-classical MHB-patterning genes.

Wg and Wnt4 provide long-range directional input to planar cell polarity orientation in Drosophila

Planar cell polarity (PCP) is cellular polarity within the plane of an epithelial tissue or organ. PCP is established through interactions of the core Frizzled(Fz)/PCP factors and although their molecular interactions are beginning to be understood, the upstream input providing directional bias/polarity axis remains unknown. Among core PCP genes, Fz is unique as it regulates PCP both cell-autonomously and non-autonomously, with the extra-cellular domain of Fz acting as a ligand for Van-Gogh (Vang). We demonstrate in Drosophila wings that Wg and dWnt4 provide instructive regulatory input for PCP axis determination, establishing polarity axes along their graded distribution and perpendicular to their expression domain borders. Loss-of-function studies reveal that Wg/dWnt4 act redundantly in PCP determination. They affect PCP by modulating the intercellular interaction between Fz and Vang, which is thought to be a key step in setting up initial polarity, thus providing directionality to the PCP process.

Gene network analysis of candidate loci for human anorectal malformations

Anorectal malformations (ARMs) are birth defects that require surgery and carry significant chronic morbidity. Our earlier genome-wide copy number variation (CNV) study had provided a wealth of candidate loci. To find out whether these candidate loci are related to important developmental pathways, we have performed an extensive literature search coupled with the currently available bioinformatics tools. This has allowed us to assign both genic and non-genic CNVs to interrelated pathways known to govern the development of the anorectal region. We have linked 11 candidate genes to the WNT signalling pathway and 17 genes to the cytoskeletal network. Interestingly, candidate genes with similar functions are disrupted by the same type of CNV. The gene network we discovered provides evidence that rare mutations in different interrelated genes may lead to similar phenotypes, accounting for genetic heterogeneity in ARMs. Classification of patients according to the affected pathway and lesion type should eventually improve the diagnosis and the identification of common genes/molecules as therapeutic targets.

Cell polarity and cystic kidney disease

Epithelial cell polarity is essential for organ development; aberrations in this process have been implicated in various diseases, including polycystic kidney disease. Establishment and maintenance of cell polarity is governed by a number of molecular processes and how these processes operate remains an interesting question. Conserved protein complexes guide both apical-basolateral polarity and planar cell polarity. In this review we discuss the recent findings that provide insights into polarity mechanisms and the intriguing crosstalk between apical-basolateral polarity and planar cell polarity, and their relationship to cystic kidney disease.

The role of primary cilia in the pathophysiology of neural tube defects

Neural tube defects (NTDs) are a set of disorders that occur from perturbation of normal neural development. They occur in open or closed forms anywhere along the craniospinal axis and often result from a complex interaction between environmental and genetic factors. One burgeoning area of genetics research is the effect of cilia signaling on the developing neural tube and how the disruption of primary cilia leads to the development of NTDs. Recent progress has implicated the hedgehog (Hh), wingless-type integration site family (Wnt), and planar cell polarity (PCP) pathways in primary cilia as involved in normal neural tube patterning. A set of disorders involving cilia function, known as ciliopathies, offers insight into abnormal neural development. In this article, the authors discuss the common ciliopathies, such as Meckel-Gruber and Joubert syndromes, that are associated with NTDs, and review cilia-related signaling cascades responsible for mammalian neural tube development. Understanding the contribution of cilia in the formation of NTDs may provide greater insight into this common set of pediatric neurological disorders.

Xenopus laevis nucleotide binding protein 1 (xNubp1) is important for convergent extension movements and controls ciliogenesis via regulation of the actin cytoskeleton

Nucleotide binding protein 1 (Nubp1) is a highly conserved phosphate loop (P-loop) ATPase involved in diverse processes including iron-sulfur protein assembly, centrosome duplication and lung development. Here, we report the cloning, expression and functional characterization of Xenopus laevis Nubp1. We show that xNubp1 is expressed maternally, displays elevated expression in neural tissues and is required for convergent extension movements and neural tube closure. In addition, xNubp1knockdown leads to defective ciliogenesis of the multi-ciliated cells of the epidermis as well as the monociliated cells of the gastrocoel roof plate. Specifically, xNubp1 is required for basal body migration, spacing and docking in multi-ciliated cells and basal body positioning and axoneme elongation in monociliated gastrocoel roof plate cells. Live imaging of the different pools of actin and basal body migration during the process of ciliated cell intercalation revealed that two independent pools of actin are present from the onset of cell intercalation; an internal network surrounding the basal bodies, anchoring them to the cell cortex and an apical pool of punctate actin which eventually matures into the characteristic apical actin network. We show that xNubp1 colocalizes with the apical actin network of multiciliated cells and that problems in basal body transport in xNubp1 morphants are associated with defects of the internal network of actin, while spacing and polarity issues are due to a failure of the apical and sub-apical actin pools to mature into a network. Effects of xNubp1 knockdown on the actin cytoskeleton are independent of RhoA localization and activation, suggesting that xNubp1 may have a direct role in the regulation of the actin cytoskeleton.

Novel mechanisms that pattern and shape the midbrain-hindbrain boundary

The midbrain-hindbrain boundary (MHB) is a highly conserved vertebrate signalling centre, acting to pattern and establish neural identities within the brain. While the core signalling pathways regulating MHB formation have been well defined, novel genetic and mechanistic processes that interact with these core components are being uncovered, helping to further elucidate the complicated networks governing MHB specification, patterning and shaping. Although formation of the MHB organiser is traditionally thought of as comprising three stages, namely positioning, induction and maintenance, we propose that a fourth stage, morphogenesis, should be considered as an additional stage in MHB formation. This review will examine evidence for novel factors regulating the first three stages of MHB development and will explore the evidence for regulation of MHB morphogenesis by non-classical MHB-patterning genes.

Apicobasal polarity and neural tube closure

During development, a flat neural plate rolls up and closes to form a neural tube. This process, called neural tube closure, is complex and requires morphogenetic events to occur along multiple axes of the neural plate. Recent studies suggest that cell and tissue polarity play a major role in neural tube morphogenesis. While the planar cell polarity pathway is known to be involved in this process, a role for the apicobasal polarity pathway has only recently begun to be elucidated. These studies show that bone morphogenetic proteins can regulate the apicobasal polarity pathway in the neural plate in a cell cycle dependent manner. This dynamically modulates apical junctions in the neural plate, resulting in cell and tissue shape changes that help bend, shape and close the neural tube.

How to form and close the brain: insight into the mechanism of cranial neural tube closure in mammals

The development of the embryonic brain critically depends on successfully completing cranial neural tube closure (NTC). Failure to properly close the neural tube results in significant and potentially lethal neural tube defects (NTDs). We believe these malformations are caused by disruptions in normal developmental programs such as those involved in neural plate morphogenesis and patterning, tissue fusion, and coordinated cell behaviors. Cranial NTDs include anencephaly and craniorachischisis, both lethal human birth defects. Newly emerging methods for molecular and cellular analysis offer a deeper understanding of not only the developmental NTC program itself but also mechanical and kinetic aspects of closure that may contribute to cranial NTDs. Clarifying the underlying mechanisms involved in NTC and how they relate to the onset of specific NTDs in various experimental models may help us develop novel intervention strategies to prevent NTDs.

Inactivation of Cdc42 in embryonic brain results in hydrocephalus with ependymal cell defects in mice

The establishment of a polarized cellular morphology is essential for a variety of processes including neural tube morphogenesis and the development of the brain. Cdc42 is a Ras-related GTPase that plays an essential role in controlling cell polarity through the regulation of the actin and microtubule cytoskeleton architecture. Previous studies have shown that Cdc42 plays an indispensable role in telencephalon development in earlier embryo developmental stage (before E12.5). However, the functions of Cdc42 in other parts of brain in later embryo developmental stage or in adult brain remain unclear. Thus, in order to address the role of Cdc42 in the whole brain in later embryo developmental stage or in adulthood, we used Cre/loxP technology to generate two lines of tissue-specific Cdc42-knock-out mice. Inactivation of Cdc42 was achieved in neuroepithelial cells by crossing Cdc42/ flox mice with Nestin-Cre mice and resulted in hydrocephalus, causing death to occur within the postnatal stage. Histological analyses of the brains from these mice showed that ependymal cell differentiation was disrupted, resulting in aqueductal stenosis. Deletion of Cdc42 in the cerebral cortex also induced obvious defects in interkinetic nuclear migration and hypoplasia. To further explore the role of Cdc42 in adult mice brain, we examined the effects of knocking-out Cdc42 in radial glial cells by crossing Cdc42/flox mice with human glial fibrillary acidic protein (GFAP)-Cre mice. Inactivation of Cdc42 in radial glial cells resulted in hydrocephalus and ependymal cell denudation. Taken together, these results highlight the importance of Cdc42 for ependymal cell differentiation and maintaining, and suggest that these functions likely contribute to the essential roles played by Cdc42 in the development of the brain.